FLAG-tag, or FLAG octapeptide, is a polypeptide protein tag that can be added to a protein using recombinant DNA technology. It can be used for affinity chromatography, then used to separate recombinant, overexpressed protein from wild-type protein expressed by the host organism. It can also be used in the isolation of protein complexes with multiple subunits.

A FLAG-tag can be used in many different assays that require recognition by an antibody. If there is no antibody against the studied protein, adding a FLAG-tag to this protein allows one to follow the protein with an antibody against the FLAG sequence. Examples are cellular localization studies by immunofluorescence or detection by SDS PAGE protein electrophoresis.

The peptide sequence of the FLAG-tag from the N-terminus to the C-terminus is: DYKDDDDK (1012 Da). It can be used in conjunction with other affinity tags, for example a polyhistidine tag (His-tag), HA-tag or Myc-tag. It can be fused to the C-terminus or the N-terminus of a protein. Some commercially available antibodies (e.g., M1/4E11) recognize the epitope only when it is present at the N-terminus. However, other available antibodies (e.g., M2) are position-insensitive.

Tumor necrosis factor receptor superfamily, member 19 (TNFRSF19), also known as TAJ-alpha or TROY, is a member of the TNF-receptor superfamily. TNFRSF19/TROY expression is detected in the pulmonary epithelium and the ductal epithelium of the prostate and parotid glands. TNFRSF19/TROY expression is detected in some adenocarcinoma cell lines that arise from this tissue. It has been shown to interact with TRAF family members, and to activate JNK signaling pathway when overexpressed in cells. TNFRSF19/TROY is capable of inducing apoptosis by a caspase-independent mechanism, and it is thought to play an essential role in embryonic development. TNFRSF19/TROY was negatively regulated by adipogenic transcription factor CCAAT/enhancer-binding proteins (C/EBP). TNFRSF19 signals activation of the Jnk pathway and induces cell death. Overexpression of TNFRSF19 also signals NFB activation, comparable and similar to that by p75NGFR. TNFRSF19/TROY is capable of activating key signaling pathways of the TNF receptor family, and its predominant expression patterns suggest that it plays a role in the growth and regulation of epithelial tissues.